Current-viewing resistors for relatively low-frequency applications typically comprise a narrow portion of a conductor on a printed circuit board, which narrow portion has a predetermined length, width and thickness and, hence, a predetermined resistance. Voltage measurements across this narrow portion of the conductor result in accurate current measurements therethrough. At relatively low frequencies, the reactance of the inductance associated with the current-viewing resistor is typically negligible and does not interfere with the attainment of accurate current measurements. However, at higher frequencies, e.g. in the RF range, the reactance of the inductance of the current-viewing resistor substantially increases. As a result, the voltage across the current-viewing resistor is no longer in phase with or proportional to the current therethrough.
Current-viewing resistors which overcome the aforementioned problems at high frequencies are presently available. Such current-viewing resistors may take the form of, for example, flat resistors or coaxial resistors. Disadvantageously, these high-frequency current-viewing resistors are generally costly and are, therefore, not practical for widespread use in commercial applications. There is, however, a present need for accurate, practical, high-frequency current sensors. For example, in an electrodeless high intensity discharge (HID) lamp ballast, a closed-loop control system such as that described in commonly assigned, copending U.S. patent application of S.A. El-Hamamsy and J.C. Borowiec, Ser. No. 631,836 filed Dec. 21, 1990, which application is incorporated by reference herein, requires accurate current sensing of the RF ballast output load current in order to generate control signals for high-efficiency operation.
Furthermore, since a current-viewing resistor typically carries the full circuit current, another desirable feature of a current-viewing resistor is a sufficiently low resistance value so that the power loss therein is negligible, while still accommodating a sufficiently large voltage drop for useful detection.